There have been used spark plugs as a member for igniting a fuel mixture gas in an internal combustion engine mounted on motor vehicles. A spark plug has a spark plug housing, an insulator, a central electrode, a ground electrode, and a terminal metal fitting. The spark plug housing has a male screw part. The male screw part is formed on an outer peripheral surface of the spark plug housing and fitted with a female part formed in a cylinder head of an internal combustion engine. The spark plug housing has a cylindrical shape.
The insulator has a cylindrical shape and is supported by the inside of the spark plug housing. The insulator has a head part projecting from the spark plug housing side toward a distal end part of the spark plug.
The central electrode is arranged at and supported by a front end part of the insulator. The ground electrode is arranged at a front end part of the spark plug housing so that a discharge gap is formed between the central electrode and the ground electrode. The terminal metal fitting is inserted into the inside of the insulator from the distal end side of the insulator. The terminal metal fitting has a main body part and a head part. The main body part of the terminal metal fitting is arranged at the inside of the insulator. The head part of the terminal metal fitting is exposed from the insulator side to the distal end side of the spark plug. The head part of the terminal metal fitting has a diameter which is greater than that of the main body part. The spark plug is electrically connected to an ignition coil through the terminal metal fitting. The ignition coil generates a high voltage and supplies it to the spark plug through the terminal metal fitting.